The invention relates to a short-circuiting and brush-lifting device for asynchronous motors equipped with a slip-ring rotor. In such a rotor the slip rings have short-circuit contacts without any axial spring mounting, which enable the slip rings to be short-circuited by a short-circuiting ring that is displaceable in the axial direction. This device also includes a device for triggering the axial displacement of the short-circuiting ring and a rotatably supported actuating ring. The actuating ring surrounds the rotor shaft and can twist between a starting position and an operating position. Guide rods (brush-actuating rods) having the same length are secured on the actuating ring by articulated. With the assistance of levers, these guide rods cause the brush-holder studs to rotate and the brushes to lift up only when the slip rings have been short-circuited.
A known short-circuiting and brush-lifting device is disclosed in the article by Moeller Werr in Electronic Engineering Handbook (Leitfaden der Elektrotechnik), vol. III Design and Strength Calculations for Electrical Machines (Konstruktion und Festigkeitsberechnungen elektrischer Maschinen), third edition, p. 122, in particular illustration 122.1. These devices are used so that after the slip-ring rotor runs up to speed over resistance, the asynchronous motor with its short-circuited rotor winding and lifted brushes can operate with low losses and without wearing out the brushes.
In this known device, the actuating ring is supported in rollers and rotatably secured to end shield. It serves to join several brush-holder studs with guide rods or lugs having the same length, which are each secured by an articulated joint to the periphery of the actuating ring and are uniformly distributed around the periphery. If the actuating ring is turned by gear teeth, which gear into a gear wheel, in the direction allocated to short-circuiting, then, the guide rods effect a rotation of the brush-holder studs with levers and lift up the brushes. A cam is connected to the gear wheel that triggers the rotation of the actuating ring, which causes the short-circuiting ring to be displaced in the axial direction. However, this short-circuiting ring contacts the short-circuit contacts of the slip rings before the brushes are lifted up and consequently short-circuits them. Since force is introduced to the short-circuiting ring at only one single point, there exists a danger that the short-circuiting ring will become skewed, because it is small in length in relation to the inside diameter. By providing a bearing arrangement on the rotor shaft with a small clearance to prevent this, this arrangement would also be sensitive to contamination, in the same way as the gear tooth engagement and the cam. Moreover, this short-circuiting and brush-lifting device is expensive, since different devices are provided for short-circuiting the slip rings and for raising the brushes.
In German Patent Number 10 05 173, an electrical machine is described having a short-circuiting and brush-lifting device. This device has a ring supported in the brush carrier housing for short-circuiting the slip rings and raising and lowering the brushes. The slip rings are short-circuited by toggle switches, which are actuated by an axially displaceable connector ring. The connector ring is moved with the assistance of a connecting link guide via a rotatable control ring. Levers for actuating the brush-lifting device are also connected to the control ring.
However, it is costly and difficult to convert a rotary motion into the axial movement of the connector ring. Moreover, there is a danger that the connector ring could become skewed.
An object of the present invention is to provide a short-circuiting and brush-lifting device which works with the lowest possible rate of wear and is insensitive to contamination.